Fuel cells produce electricity through electrochemical reaction and have been used as power sources in many applications. Fuel cells can offer significant benefits over other sources of electrical energy, such as improved efficiency, reliability, durability, cost and environmental benefits. Fuel cells may, eventually be used in automobiles and trucks. Fuel cells may also power homes and businesses.
There are several different types of fuel cells, each having advantages that may make them particularly suited to given applications. One type is a proton exchange membrane (PEM) fuel cell, which has a membrane sandwiched between an anode and a cathode. The membrane and respective electrodes together in an assembly are referred to as a membrane electrode assembly (MEA). To produce electricity through an electrochemical reaction, hydrogen (H2) or hydrogen containing gas is supplied to the anode side half-cell via an anode flow field and air or oxygen (O2) is supplied to the cathode side half-cell via a cathode flow field.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). Because the membrane is proton conductive, the protons are transported through the membrane. The electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+) and electrons (e−) are taken up to form water (H2O). Parasitic heat is generated by the reactions and must be regulated to provide efficient operation of the fuel cell stack.
Fuel cell stacks include reactant flow fields and coolant flow fields. The reactant flow fields distribute anode and cathode reactant fluids across the individual cells of the fuel cell stack. The coolant flow fields distribute a heat transfer (coolant) fluid to regulate the operating temperature of the fuel cell stack. In the case of the reactant flow fields, the anode and cathode reactant fluids are distributed as gas phase fluids. In the case of the coolant flow fields, the heat transfer fluid is distributed as a liquid phase fluid.
Under certain operating conditions, a liquid phase fluid can form in the reactant flow fields. The liquid phase fluid impedes flow of the gas phase reactant fluids, which can result in unstable operation of the fuel cell stack. Such a situation typically occurs when consistently operating the fuel cell stack at a low load level. Traditionally, the stability issues are mitigated by applying higher anode and/or cathode stoichiometry to avoid unstable operation. This presents a significant disadvantage, however, in that a high percentage of the reactant is wasted. As a result, system efficiency is decreased.
Gas and/or vapor phase fluids can also form in the coolant flow fields. The gas phase fluid impedes flow of the liquid phase heat transfer fluid, which can result in localized temperature increases or hot spots within the fuel cell stack. These hot spots, reduce the durability of the fuel cell stack and can result in localized damage to the fuel cell stack. The presence of such hot spots in the stack or in a particular cell can be identified by some increase in temperature and/or in cell resistance, which leads to some decrease in cell voltage.